This invention relates to an antenna unit and, more particularly, to an antenna for an ultra wideband (UWB).
The UWB technology means an ultra wideband radio technology like its name and is defined as any radio technology having a spectrum that occupies a bandwidth greater than 25 percent of the center frequency, or a bandwidth of at least 1.5 GHz. In a word, the UWB technology is technology for communicating using short pulses (normally each having a pulse width of 1 ns or less) of ultra wideband so as to start a revolution in radio technology.
A crucial difference between a conventional radio technology and the UWB technology is the presence or absence of a carrier wave. The conventional radio technology modulates a sinusoidal wave having a frequency called the carrier wave using various methods to transmit and receive data. On the other hand, the UWB technology does not the carrier wave. In the manner which is written in definition of the UWB technology, the UWB technology uses the short pulses of the ultra wideband.
Like its name, the UWB technology has a frequency band of the ultra wideband. On the other hand, the conventional radio technology has only a narrow frequency band. This is because it is possible for the narrow frequency band to put electric waves to practical use. The electric waves are a finite resource. The reason whey the UWB technology is widely noticed in spite of the ultra wideband is output energy of each frequency. The UWB technology has a vary small output each frequency in place of a wide frequency band. Inasmuch as the output of the UWB technology has magnitude so as to be covered with noises, the UWB technology reduces interference with other wireless spectra. In the United States, the Federal Communications Commission (FCC) has mandated that UWB radio transmissions can legally operate in range from 3.1 GHz up to 10.6 GHz, at a limited transmit power of −4.1 dBm/MHz.
In addition, antennas basically use a resonance phenomenon. The antenna has a resonance frequency which is determined by its length, it is difficult for the UWB including a lot of frequency components to make the antenna for UWB resonate. Accordingly, the wider the frequency band of the electric wave to be transmitted is, the more difficult it makes a plan for the antenna for UWB.
For example, patch antennas are known as small-sized antennas in the art. As one of the patch antennas, a compact plane patch antenna is disclosed, for example, in JP 07-094934 A. According to JP 07-094934 A, the compact plane patch antenna has high infrequency temperature characteristics and high reliability by using magnesium titanate ceramic having comparatively high dielectric constant as a main material for a dielectric material and adding the proper quantity of lithium niobate, alumina, manganese oxide, etc., individually or their combination at ions to the main material to mold the antenna. In addition, a patch antenna device capable of coping with a plurality of frequencies is known, for example, in JP 10-190347 A.
However, the patch antennas are unsuitable for the UWB antennas because the patch antennas have no wideband.
On the other hand, Taiyo Yuden Co. Ltd. has successfully developed a very miniaturized ceramic chip antenna having a size of 10×8×1 mm for ultra wideband applications. Since UWB technology was released by the FCC for commercial use, it has been hailed as the short-range wires-communication standard of the future. For one thing, it promises to simultaneously provide a high data rate and low power consumption. By sending very low-power pulses below the transmission-noise threshold, UWB also avoids interference. By developing the antenna, is has become the responsibility of the wireless industry to help UWB make the transition from military applications to widespread commercial use for connecting at a very high speed data between digital devices such as PDP (plasma display panel) television, a digital camera, or the like.
In addition, such a UWB antenna can be used for various purposes such as Bluetooth (registered trademark), wireless LAN (local area network), or the like.
Bluetooth (registered trademark) technology is a cutting-edge open specification that enables short-range wireless connections between desktop and notebook computers, handhelds, personal digital assistants, mobile phones, camera phones, printers, digital cameras, handsets, keyboards and even a computer mouse. Bluetooth wireless technology uses a globally available frequency band (2.4 GHz) for worldwide compatibility. In a nutshell, Bluetooth technology unplugs your digital peripherals and makes cable clutter a thing of the past.
The wireless LAN is a LAN using a transmission path except for a wire cable, such as electric waves, infrared rays, or the like.
In the manner which is described above, the conventional antenna such as a patch antenna is disadvantageous in that it is difficult to widen the band and wave distortions (wave expansion) occur.
On the other hand, the present co-inventors have been developed an antenna unit of a direct-feeding type and this assignee already file an application at Sep. 18, 2003 as Japanese Patent Application No. 2003-325858 which corresponds to European Patent Application No. 04253764.7 (Jun. 23, 2004) and to U.S. patent application Ser. No. 10/874,910 (Jun. 22, 2004). However, the direct-feeding has a poor matching characteristic and a large return loss.